Fillet question - is epoxy necessary on outside Fin-BT for G,H?

[Alan R]

Hey @Mugs914 ... I would venture a guess that's not your first. Do you have a lot of planes, and also.. do you have a pilot license and fly too?

 

[Not Quite Nominal]

The outside BT to fin joint isn't loaded in shear or tension (the usual failure modes of glue), so you don't even need to fillet it.

 

[grouch]

I only build with wood glue now as all my kits are paper. F-J no worries at all. I'd fly larger with wood glue too but don't have anything larger than a 38mm mount at the moment.

 

[afadeev]

What others have said about TBII locking couplers in place is true. I rely on quickness and occasionally a guide to get them into place before it locks up, but have had some close calls.

All true, and the primary reason why I almost always rely on epoxy to glue motor mounts (MMT) inside the airframe.
The other aspect of TBII is that it shrinks everything it touches when drying, including the circumference of your paper airframe tube around the MMT centering rings. This will leave you with a circular indentation on the airframe around the area where the centering rings were glued.
Structurally, not a problem, but cosmetically suboptimal.

I use TBII (its what I use in my shop) for all initial bonds on cardboard and wood rockets even the first layer of fin fillet is usually TBII, then I start building my fillets with Q&T on top of that.

I didn't have any TB Q&T on hand (just ordered some), so always resort to epoxy for fin fillets. Either BSI 30 min (when small quantities are desired), or West Systems 105/205. WS works way better, spreads more evenly and dries crystal clear. But metering small quantities with WS pumps is a PITA, and the smallest volume I can reliably mix is ~3.5g (1/8 fl oz), which leads to waste.

For what it's worth, here are the pictures of fin fillets after 3 layers of TBII (after which I gave up), and with 30 min BSI epoxy fillet on top.
Even after multiple layers with some sanding, I could not get TBII to dry flat and even. It would look perfect wet, and then boulder while drying. No such problems with epoxy:

 

[Nytrunner]

This will leave you with a circular indentation on the airframe around the area where the centering rings were glued

The funny thing is, I dont doubt this happens since so many folks cite it. It just hasnt happened to me lol

 

[neil_w]

For what it's worth, here are the pictures of fin fillets after 3 layers of TBII (after which I gave up), and with 30 min BSI epoxy fillet on top.
Even after multiple layers with some sanding, I could not get TBII to dry flat and even. It would look perfect wet, and then boulder while drying. No such problems with epoxy:

Cosmetically nice fillets with TBII are, as you've seen, very difficult and generally not worth the trouble IMHO. Like Rich, I have settled on a single application of TBII to really soak into the joint and fill any gaps, and then layer on top with Q&T to build it up and get a smoother look.

Well-done epoxy fillets will always look better and require only one pass to apply, but they are usually overkill and add unnecessary weight on smaller rockets.

 

[gldknght]

I use WEST System epoxy and use their phenolic microballoons to thicken it up to make my fin fillets. Very lightweight and it doesn't run.

On another post someone mentioned these & it really works well.

https://www.amazon.com/gp/product/B0771DY1LK/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1

I think I've used West system with microballoons on every one of my high power builds. It's almost too easy to get a good looking fillet.

 

[Alan R]

For what it's worth, here are the pictures of fin fillets after 3 layers of TBII

Thats a weird fin set. What kit is that?

 

[neil_w]

Thats a weird fin set. What kit is that?

That's an Interceptor. Your rocket geek credentials are hereby revoked.

 

[afadeev]

That's an Interceptor.

Indeed.
Estes #1250 (Interceptor), the still in production classic looking kit that goes way back a few decades.
https://www.acsupplyco.com/estes/explorer.htm
Also known as the 18mm brother of Estes #1350 (Interceptor E), the 24mm OOP version of the same (next in my build pile).
That one will look something like the follow, unless I screw it up:
https://www.ebay.com/itm/Accur8-Bla...Estes-Interceptor-E-Model-Rocket/222006187799

 

[Alan R]

That's an Interceptor. Your rocket geek credentials are hereby revoked.

Indeed.

Estes #1250 (Interceptor), the still in production classic looking kit that goes way back a few decades.

I will just hang my head in shame now and hope that no one else ever sees this

 

[Weldo]

I will just hang my head in shame now and hope that no one else ever sees this

Too late! The internet never forgets! EVER!

Haha, I also had no idea what it was! I like those kind of kits though. The ones that look like jets really pique my interest. I built an Apogee SR-72 Darkbird a few months ago and It looks so cool! Flies well to boot!

Doesn't someone make a kit called Snarky that looks kind of like that? I've also seen a similar one in some Apogee vids but don't know what model it is. Kinda looks like an F-104 Starfighter with the shark teeth and everything...

 

[afadeev]

Haha, I also had no idea what it was! I like those kind of kits though. The ones that look like jets really pique my interest. I built an Apogee SR-72 Darkbird a few months ago and It looks so cool! Flies well to boot!

In that case, I think you will probably have fun with the Estes SR-71 kit #1942.
It's OOP, but can still be found on eBay.
I had just built it the other week. Paper shrouds were a nice challenge, and so were the old waterslide decals, but overall, I had fun with it.
Structural gluing was done with TBII, although I would revisit that decision for attaching paper fuselage shroud to the main aiframe tube. TBII had somewhat deformed the glued areas after drying, and those required a fair amount of filling and sanding afterwards.
All the fin fillets were done with either 30 min BSI, or West Systems epoxy (can't recall now):
https://www.rocketryforum.com/threads/coronavirus-work-from-home-build-thread.158551/#post-1984200

Doesn't someone make a kit called Snarky that looks kind of like that? I've also seen a similar one in some Apogee vids but don't know what model it is. Kinda looks like an F-104 Starfighter with the shark teeth and everything...

Dynastar makes a Snarky #5030 kit with 24mm motor mount.
It, and other Dynastar kits, can be found here: https://www.erockets.biz/dynastar-flying-model-rocket-kit-snarky-dyn-5030/

 

[Weldo]

Cool, thanks!

 

[Mugs914]

Hey @Mugs914 ... I would venture a guess that's not your first. Do you have a lot of planes, and also.. do you have a pilot license and fly too?

Sorry Alan, I missed this post somehow!

FAR from my first, actually! I think I built my first stick and tissue model when I was in the third grade or so, and have been building and flying model planes on and off ever since. I mostly fly control line and RC until a few years ago when I re-discovered free flight. I do have quite a few planes of all types in the ole workshop, a few boats too (Used to race RC hydros).

One of the reasons my builds take so long is because I suffer from HRADD (Hobby-related Attention Deficit Derangement) and tend to bounce around between projects a lot. Add in a few non-modelling hobbies and some never get done!

And yes, I used to fly, but haven't in years. WAY too expensive!

 

[Alan R]

And yes, I used to fly, but haven't in years. WAY too expensive!

I was just testing a theory. I have known more than one pilot who liked to build airplanes when they were not actually flying themselves.

 

[Greg Furtman]

One of the reasons my builds take so long is because I suffer from HRADD (Hobby-related Attention Deficit Derangement) and tend to bounce around between projects a lot. Add in a few non-modelling hobbies and some never get done!

 

[DaHabes]

For great fillets on a rocket, I use Rocketpoxy, even on some (bigger) LPR. They don't add a lot of weight if you keep them small for looks.

Use these:

https://www.amazon.com/Fondant-Flow...nd+tools&qid=1589198119&s=home-garden&sr=1-91

to make the fillets following this videos instruction:

https://www.badassrocketry.com/badassvideos?pgid=k8hglni52-997dc215-dfa5-4082-9087-53086d4ad024

 

[Zman1961]

Mike, great post, thank you! I am assuming that you would mix a new batch of expoy for each pair of fillets, and that it would take an evening to do each pair.

 

[DaHabes]

Yes. Since I'm retired, on a smaller rocket I can get two sets done per day if I'm at all organized! If the fillets are small Rocketpoxy will be pretty well set after 5-6 hours. On bigger birds with bigger fillets, I get one set done per day.